The chemical and physical properties of aerosol particles that determine their impact on climate are determined in part by their composition, size, shape, and morphology. We have characterized the morphology of submicron aerosol particles composed of organic compounds mixed with ammonium sulfate that undergo liquid-liquid phase separation. Surprisingly, the morphology of some systems is dependent on their size, where small particles are homogeneous and large particles are phase separated. The inhibition of phase separation at the nanoscale is due to an underlying thermodynamic, nanoscale effect. Specifically, small particles are unable to overcome the activation barrier needed to form a new phase. We have subsequently demonstrated that the cloud condensation nucleus activity of these particles is dependent on their morphology. This result indicates that morphology of particles can impact their ability to form cloud nuclei, and therefore affect climate.